The present invention relates to an optical signal control method and apparatus. In particular, it relates to automatic control of polarisation of optical signals.
In general, optical waveguides do not preserve the polarisation state of light emerging from a length of waveguide: the polarisation varies with time.
A single mode fibre having circular symmetry about the axis of the core allows propagation of two orthogonally polarised modes. The fibre behaves as a birefringent medium due to differences in propagation speeds of the two modes resulting from anisotropy of the refractive index of the fibre core. A number of birefringence-inducing mechanisms are known. For example, a non-circular core introduces linear birefringence: the smaller transverse dimension of the fibre core becomes the fast axis of birefringence. Changing the electro-optic index, for example by applying an asymmetrical transverse stress, also induces linear birefringence, as does the application of a transverse electric field Linear birefringence may also be controlled by applying a voltage to electro-optic material such as lithium niobate. Alternatively, circular birefringence may be induced by means of a magnetic field along the length of the fibre (Faraday effect).
In many circumstances, it is desirable to be able to control the state of polarisation (SOP) of the modes in a fibre. One example is in coherent detection systems, where the polarisation states of the incoming signal and the local oscillator must be the same. It is therefore important to be able to control the SOP of one of the signals. Ideally, it should be possible to match any polarisation state to any other state, where both the initial and final states may vary.
Polarisation states can be represented on the Poincare sphere. This representation is fully described in published literature, e.g. Rashleigh: "Origins and Control of Polarisation Effects in Single Mode Fibres", J Lightwave Technology Vol. LT 1 No 2 June 1983 p. 312-331. Any general elliptical polarisation state such as shown in FIG. 1, where .psi.=.+-.arc tan b/a, is represented on the sphere by a single point S as shown in FIG. 2.
Horizontal and vertical polarisation states are represented by H and V respectively, and all linear states lie on the great circle HPVQ, where the latitude is zero. P and Q represent polarisation at .+-..pi./4 to the L and V states. L represents left hand circular polarisation and R right hand circular polarisation. Any state of polarisation is represented by a unique point on the sphere, where .psi.i and .phi.i of a particular elliptical polarisation state are represented by co-ordinates 2 .psi.i and 2 .phi.i on the sphere.
Birefringence causes a change in polarisation state from S to S' and thus a rotation about an axis passing through the centre of the sphere, through an angle which depends on the magnitude of the birefringence. Linear birefringence causes rotation about an axis lying in the plane HPVQ of FIG. 2.
Various methods have been proposed for controlling the SOP of a waveform propagating in single mode fibre. In a coherent optical communications system, a slight polarisation mismatch between the incoming signal and local oscillator signal causes a significant fall in received signal. The polarisation states of both signals may vary with time so if only one polarisation state is controlled, the controller must be able to transform any polarisation state S into any derived state S', where S, S' can lie anywhere on the Poincare sphere. Two birefringent elements in series are inadequate to transform S to S' for every S, S', but three elements may be adequate under certain circumstances. In practice a stress inducing birefringent element, for example, cannot apply a greater and greater stress to the fibre, or the fibre will break. It is therefore necessary to be able to reset or adjust birefringent elements, meanwhile maintaining the transformation of the SOP of the signal from the initial to final states, however those states may vary. Methods of resetting have been proposed, involving the introduction of further birefringent elements.
In Electronics Letters, Vol 22 No 2, Jan. 16th 1986, page 100, Rysedale proposes polarisation transformation using four birefringent elements in series. The first, third and fourth elements are linearly birefringent, causing rotation of the SOP on the Poincare sphere about axes x, y and x respectively (x, y orthogonal), and the second element is circularly birefringent, causing rotation about the axis LR.
This arrangement could, if suitably programmed, continuously alter a fixed SOP S to a final varying SOP S'. This is achieved without serious loss of intensity by resetting the birefringence of the third and fourth elements.
In Proceedings of the Sixth European Symposium on Optoelectronics (OPTO 86), Paris, May 12-16 1986, a paper by R. Noe and G. Fischer describes an alternative approach involving five electromagnetic linear birefringent elements ("fibre squeezers") in series. The first, third and fifth squeezers are aligned to induce birefringence having the same fast axis, and interposed second and fourth squeezers are aligned at 45.degree. to the others. Thus, a series of five rotations takes place on the Poincare sphere about the axes QP (three rotations) and HV (two rotations). A similar arrangement involving only four squeezers is described in Electronics Letters, Vol. 22, No. 15, July 17th 1986, p. 772-3. This can transform an initially horizontally polarised signal into any desired state of polarisation. The first two transformations (d.sub.1 and d.sub.2) are effectively kept in reserve, d.sub.3 and d.sub.4 achieving the necessary transformations themselves, until a range limit is reached. If d.sub.3 reaches a range limit, an exchange is carried out using d.sub.1, to bring d.sub.3 back to a predetermined point within its range. Similarly, d.sub.2 can be used for exchange with d.sub.4 when it reaches its range limit.
The present invention can provide practical and robust means for polarisation control or matching. Apparatus according to the invention may comprise three or four variable birefringent means, such as linear birefringent elements, arranged to rotate the state of polarisation sequentially about respective axes on the Poincare sphere, where the axes are preferably substantially orthogonal.
The use of four elements enables a desired transformation between two time varying polarisation states to be maintained (which covers "adhered to" in the case of a varying requirement) without the birefringence of any element falling outside its working limits. At selected or appropriate times, one element is subjected to an adjustment procedure to vary its birefringence. This enables elements to be `unwound`--i.e. their birefringences increased or decreased - by any desired amounts to keep all elements within their limits. The adjustments may be frequent small ones, or large ones (e.g. 2.pi. or more on the Poincare sphere) if elements are allowed to approach their working limits. The adjustment procedure may, for example, be carried out sequentially on each element in turn, or by selecting the element nearest the limits; there are many other possibilities. With four elements, it will generally be possible to unwind the outer two elements, if required, to any desired extent. Depending on the algorithm selected for control, it may never be necessary to unwind, or select for adjustment, the inner two elements. If the circumstances are such that the inner elements do require unwinding, they can be, although it may be necessary to impose a delay until the two varying polarisation states (initial and final states) are appropriately located on the Poincare sphere for unwinding to continue.
Similarly, with three elements, it is possible to transform between two polarisation states where one of the states may vary with time. In this case, it may only be necessary to unwind the element remote from the fixed state.
Arrangements according to the invention can be extremely flexible, and the controller itself can operate in a more straightforward manner than in known prior art proposals.
The various aspects of the present invention are set forth in the appended claims, and embodiments of the invention are described below.
These provide particularly simple and convenient ways of controlling polarisation. The number of birefringent elements required is fewer, in some cases, than in prior art systems, and the procedure is generally simpler and cheaper to operate. It can also be more robust and flexible.
Throughout the specification and claims, it is to be appreciated that where reference is made to a variable birefringent element carrying out a rotation of a given angle, the given angle is intended to refer to the net rotation (unless the context indicates otherwise). Such a net rotation could for example be achieved by rotating by the given angle plus or minus any multiple of 2.pi.. Also, references are made throughout to orthogonal axes of rotation (a, b, x, y etc). In practice, true orthogonality is hard to achieve and variations can be accommodated without significant loss of matching/signal.